For diagnosis of patients with vertigo and/or disequilibrium, it is important to analyze eye movements in terms not only of the horizontal and vertical components but also of the torsional component as well. However, generally practiced electronystagmography (ENG) permits analyses of only the horizontal and vertical components of eye movements.
Other conventional methods of measuring eye movements include high-velocity photography and video monitoring. The high-velocity photography method takes pictures of iris striations (positions of the eyeball not at the eyeball center) of an eyeball using a high-velocity camera, develops the pictures, and then sequentially measures striation movements relative to the center of a pupil by applying a scale to each printed picture, so that the torsional angle and velocity of the eyeball are calculated.
The video monitoring method, on the other hand, reproduces iris striation movements imaged by a video camera on a monitoring unit, sequentially measures the movement of striations relative to the center of a pupil by applying a scale on the monitoring unit on a single field or frame basis, so that the torsional angle and velocity of the eyeball are calculated.
Also known is a method of measuring the torsional component of eye movements by tracing an eyeground pattern, instead of tracing the movement of iris striations, using the high-velocity photography and the video monitoring method.
The high-velocity photography method requires that after having taken pictures, the film be developed and printed, thereby entailing much time and labor. Also, a cumbersome operation of applying a scale to each printed picture must be undergone.
The video monitoring method also requires that eye movements be measured by applying a scale to each image visualized on the monitoring unit and then calculated, thereby being time-consuming in measurement as in the high-velocity photography.
Thus, neither the high-velocity photography nor the video monitoring method can produce measurement results quickly, and it is for this reason that they have not been suitable for clinical applications.
In the technique for measuring eye movements by tracing an eyeground pattern, movement of the center of the pupil or contraction of the pupil hides and blinds the eyeground pattern. Thus, this technique is more difficult compared to the techniques for tracing the iris striations.